Electrocyclization reaction of higher conjugated polyenals: photochemical behaviors of retinal (vitamin A1 aldehyde) homologues.

During the studies on a photoreaction of retinal, involving various kinds of (Z)-(E) isomerization, a heretofore unknown photoproduct of retinal was isolated in a pure state and was characterized unambiguously. Thus, direct irradiation of all-(E)-retinal (I) and of all-(E)-beta-ionylidenecrotonaldehyde (II) in acetonitrile solution gave the corresponding 6e-electrocyclized photoproducts, (III) and (IV), both via the possible 7-(Z)-isomer intermediates of the parent conjugated polyenals. Unlike the lower members in the retinal series, it was also confirmed that sigmatropic rearrangement or photo-Diels-Alder reaction hardly proceeds in these higher members of the series mentioned above.